The present invention relates to a charging member which is incorporated in an image forming apparatus such as an electrophotographic copying machine and an electrostatic recording apparatus and electrifies an image forming object, and more preferably to a charging unit of a magnetic brush type.
Heretofore, a corona charging unit has generally been used for charging of an image forming object such as a photoreceptor drum in an image forming apparatus of an electrophotographic type. In the corona charging unit, high voltage is impressed on a corona wire to generate an intensive electric field around the corona wire for gas discharge, and charged ions generated on that occasion are adsorbed on the image forming object which is thus charged.
The conventional corona charging unit used in an electrophotographic image forming apparatus as those mentioned above does not come into contact with an image forming object mechanically for charging, and therefore, it has an advantage that it does not hurt the image forming object when charging. However, it has disadvantages that there are risks of an electric shock and a leak, while ozone caused by gas discharge is harmful for the human body and it shortens a life of an image forming object. Further, charging voltage by means of a corona charging unit is intensely affected by temperature and humidity and therefore is unstable, noise is further produced by high voltage in the corona charging unit, and a period of time of 5 seconds or more is needed for obtaining stable charging voltage after inputting high voltage, which are serious disadvantages when an electrophotographic image forming apparatus is utilized as a communication terminal and an information processing apparatus.
Main cause for the various disadvantages of the corona charging unit mentioned above lies in that charging is carried out mainly through gas discharge.
Accordingly, as a charging unit capable of charging the image forming object without requiring high voltage gas discharge as in a corona charging unit and without hurting the image forming object mechanically, Japanese Patent Publication Open to Public Inspection No. 133569/1984 (hereinafter referred to as Japanese Patent O.P.I. Publication) discloses a charging unit wherein magnetic particles are adsorbed on a cylindrical magnetic-particle-carrier housing therein magnets for forming a magnetic brush thereon and the magnetic brush rubs the surface of an image forming object for charging while D.C. bias voltage is impressed.
Since the magnetic brush mentioned above is a flexible brush composed of magnetic particles, it can charge without hurting an image forming object and has an advantage over other contact type charging units such as a fur brush charging unit and a charging unit employing a conductive and elastic roll. However, even when the magnetic brush charging unit mentioned above is used, uniform charging has not necessarily been attained.
Accordingly, magnetic brush charging methods wherein A.C. bias voltage containing D.C. components is applied on a magnetic brush for charging are disclosed in Japanese Patent O.P.I. Publication Nos. 21873/1992 and 116674/1992 which describe that the aforesaid A.C. bias voltage is impressed and thereby uniform charging can be given to an image forming object.
Even in the case of the charging method described in the aforesaid official gazette, breakdown is caused, magnetic particles are stuck on an image forming object or sufficient charges are not injected in magnetic particles, resulting in occurrence of uneven charging, if appropriate peak-to-peak voltage V.sub.P-P is not set.
However, appropriate peak-to-peak voltage V.sub.P-P is varied by environmental change. Therefore, when setting to the specific peak-to-peak voltage V.sub.P-P, magnetic particles are caused to have high electric resistance so that magnetic particles tend to adhere to an image forming object when temperature and humidity are low, while when temperature and humidity are high, magnetic particles are caused to have low electric resistance, resulting in occurrence of breakdown, which have been problems.
In Japanese Patent O.P.I. Publication No. 267667/1989, there is further disclosed a technology wherein bias voltage composed of D.C. constant voltage and A.C. voltage in which peak-to-peak voltage V.sub.P-P is set so that specific A.C. components therein may be constant current is impressed on a charging unit. However, even when A.C. components are made to be constant current, optimum peak-to-peak voltage V.sub.P-P for an image forming object varies depending on difference of variation in an image forming object and a charging member themselves, and therefore satisfactory charging can not sometimes be obtained, which has been a problem.